. net framework 4 breaks Central administration and a few Web Apps- SP 2013

After weeks of everything working fine. Central admin and a few Webapps break down suddenly in SP2013

Was getting error:

System.IO.FileLoadException: Loading this assembly would produce a different grant set from other instances. (Exception from HRESULT: 0x80131401)

I set the trust level="Full" originUrl="" legacyCasModel="false" in web.config for Central admin and the broken Webapps. It started working.

Wanted to know the main reason for making this web.config change. If any custom dll's would have impacted it was fine. But I had to make the changes for Central admin to work.

Any help will be greatly appreciated !

Regards, Sushil

Setting legacyCasModel="false" is not supported according to: this.